Modify the sudoers file to lecture RO users once

Debian changed the defaults of the sudo package to never lecture the
user when using an unauthorized sudo command, which breaks our use case
of lecturing once. Add a line to lecture once, which is the old
defaults.

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

files/image_config/sudoers/sudoers

@@ -11,6 +11,7 @@ Defaults        env_reset
 Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
 Defaults        env_keep += "SONIC_CLI_IFACE_MODE"
 Defaults        env_keep += "VTYSH_PAGER"
+Defaults        lecture = once
 Defaults        lecture_file = /etc/sudoers.lecture
 
 # Host alias specification